PROJECT SUMMARY Rhabdomyosarcoma (RMS) is a devastating malignancy of muscle that is diagnosed in hundreds of children and adults annually in the USA. Survival rates are less than 30% in patients with unresectable, metastatic, or relapsed RMS, with continued tumor growth being maintained by a small number of self-renewing, tumor- propagating cells (TPCs). Yet, to date, targeted approaches to kill TPCs or to differentiate them into non- proliferative, differentiated RMS cell types have not been developed. The long-term goal of our work is to uncover therapeutically relevant pathways that drive RMS growth through their regulatory effects on TPCs. The overall objective of this application is to determine the extent to which non-canonical Wnt/Planar Cell Polarity (Wnt/PCP) signaling regulates TPC self-renewal and can be targeted to inhibit RMS growth. Our central hypothesis is that Van Gogh-like 2 (Vangl2), a core regulator of the Wnt/PCP signaling pathway, modulates self-renewal and growth of RMS TPCs. We also hypothesize that Vangl2 expression is confined to TPCs and can be used to isolate and characterize these cells. Our preliminary data indicate that Vangl2 and Wnt/PCP signaling regulate TPC self-renewal in both zebrafish and human RMS. VANGL2 inactivation leads to reduced TPC number, decreased tumor cell growth, and elevated differentiation in human RMS cells both in vitro and in vivo using mouse xenografts. Fluorescent transgenic zebrafish models of embryonal RMS also showed that Vangl2 expression enriches for self-renewing TPCs, providing novel approaches to dynamically visualize these cells in live animals and to quantify effects of altering Wnt/PCP signaling on self-renewal. The rationale for our work is that VANGL2 is active in a vast majority of human RMS and is required for continued tumor growth and self-renewal, suggesting that therapeutic strategies based on VANGL2 inhibition would benefit a large fraction of high-risk patients. Aim 1 will assess the role for Wnt/PCP signaling in RMS growth and self-renewal in a fluorescent-transgenic zebrafish model and patient-derived xenografts, testing our hypothesis that Vangl2 and the Wnt/PCP pathway regulate self-renewal and expansion of TPCs in RMS. Aim 2 will characterize Vangl2 as a marker of TPCs in both zebrafish and human RMS, providing unprecedented access to dynamically visualize roles for the Wnt/PCP pathway in regulating self-renewal and cell fate choices following cell division. Aim 3 will elucidate the effector pathways downstream of VANGL2 and Wnt/PCP signaling, testing our working hypothesis that VANGL2 drives self-renewal through the activation of RHOA small GTPase signaling. Our work will uncover the molecular pathways by which VANGL2 and the Wnt/PCP pathway drive human RMS growth and self-renewal. Such insights will provide new biomarkers for assessing drug effects on TPCs and will likely identify novel drug targets beyond VANGL2 for the treatment of RMS. Our work is predicted to have a large positive translational impact, advancing our understanding of processes that control self-renewal in cancer and providing pre-clinical efficacy of targeting VANGL2 to suppress growth of patient derived xenografts.